The following description is used to describe the background and products, materials and production methods that may comprise specific parts of preferred embodiments in the disclosure of this invention.
a) Laminate Floorings.
The majority of all laminate floors are produced according to a production method generally referred to as Direct Pressed Laminate (DPL). Such laminated floors have a core of 6-12 mm fibreboard, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate, plastic, paper or like materials.
The surface layer of a laminate floor is characterized in that the decorative and wear properties are generally obtained with two separate layers of paper, one above the other. The decorative layer is generally a printed paper and the wear layer is a transparent overlay paper, which comprises small aluminium oxide particles.
The decor paper is the most critical of the lamination papers as it gives the visual appearance of the laminate. The decor paper weight is generally in the range of 60-150 g/m2.
The overlay paper is generally thinner with a weight of about 20-50 g/m2 and is made of pure cellulose, which is based on delignified pulp. The overlay paper becomes almost completely transparent after lamination and the appearance of the decor paper is visible.
Thicker overlay papers with a considerable amount of aluminium oxide particles may give a high wear resistance. The disadvantage is that they are less transparent and the decorative pattern is covered by a grey layer that disturbs the printed pattern.
Printing of decorative papers is very cost efficient. Rotogravure presses with printing cylinders that may have a width of 3 metres and that may run with a speed of up to 600 m/min are used. The printing cylinders are generally produced by conventional mechanical engraving. Recently digital laser engraving has been introduced which allows faster décor development and provides a better décor quality. Solvent-free inks with organic pigments are often used and excess ink is re-cycled.
The printed decorative paper and the overlay are impregnated with melamine formaldehyde resins, generally referred to as melamine resins, and laminated to a HDF core in large discontinuous or continuous laminate presses where the resin cures under high heat (about 170° C.) and pressure (40-60 bars) and the papers are laminated to the core material. An embossed press plate or steal belt forms the surface structure. Sometimes a structured paper is used as a press matrix. The embossing is in high quality floors made in register with the design. The embossing depth is limited to 0.1-0.2 mm (100-200 micron).
Laminated floors may also be produced with direct printing technology. One advantage is that the pressing operation may be avoided and that no printed papers are needed to provide a decorative surface. Hydro printing inks are used to print the décor by a multicolour printing press with rollers onto a pre-sealed core and the print is covered with a protective transparent wear layer that may be an overlay, a plastic foil or a lacquer. The production process is rather complicated and is only cost efficient in very large production volumes.
Direct printing technology may be replaced with digital printing technology that is much more flexible and small production volumes can be economically manufactured. The difference between these two methods is mainly the printing step where the printing rollers are replaced by a digital non-contact printing process.
Digital printing may also be used to print on a paper sheet that is used in conventional laminate production and laminated under heat and pressure. The printing may be made prior or after impregnation. Such printing prior to impregnation is complicated since paper may swell and shrink during the printing and impregnation step and small quantities are not cost efficient to impregnate. Printing after impregnation on a melamine impregnated paper is very difficult since pigments applied on a melamine surface float during the pressing step when the melamine resin is in a liquid state. Such problems may partly be solved with a method where a raw paper, preferably comprising a base colour, is applied and fixed to the core prior to printing and impregnated paper or melamine powder is applied under and/or over the raw paper such that the resins from the impregnated papers penetrate into the raw paper during the pressing step.
Laminate floors may also have a surface of paper foils or plastic foils and such foil materials may also be printed digitally. A protective wear resistant transparent layer that generally is a polyurethane lacquer is used to covers the printed décor.
b) Powder Based Floors (WFF)
Recently new “paper free” floor types have been developed with solid surfaces comprising a substantially homogenous powder mix of fibres, binders and wear resistant particles hereafter referred to as WFF (Wood Fibre Floor).
The powder mix may comprise aluminium oxide particles, melamine formaldehyde resins and wood fibres. In most applications decorative particles such as, for example, colour pigments are included in the mix. In general, all these materials are applied in dry form as a mixed powder on a HDF core and cured under heat and pressure to a 0.1-1.0 mm solid layer. The powder is, prior to pressing, stabilized with moisture and IR lamps such that it forms an upper skin layer similar to a paper layer and this prevents the powder from blowing away during pressing. Melamine formaldehyde powder and wood fibres may be replaced by thermoplastic particles.
Several advantages over known technology and especially over conventional laminate floorings may be obtained such as increased wear and impact resistance, deep embossing, increased production flexibility and lower costs. An embossing depth of 0.2-0.7 mm may easily be reached.
Powder technology is very suitable to produce a decorative surface layer, which is a copy of stone and ceramics. In the past it was more difficult to create designs such as, for example, wood decors. However, recently digital powder printing has been developed and it is possible to create very advanced designs of any type by injecting ink into the powder prior to pressing. Problems related to paper impregnation may be completely eliminated since no impregnation is required. The surface structure is made in the same way as for laminate flooring by a structured press plate, a steal belt or an embossed matrix paper that is pressed against the powder. A major advantage compared to the other digital printing technologies is that the powder provides a base colour and no protective layer is needed above the print since the ink may penetrate into the powder. The penetration is however rather limited since the ink drops will be bonded to the first particle that they hit, mainly the wood fibres. Increased wear resistance may be reached if several printed powder layers are applied on each other or if a powder overlay is used as a protective layer applied over the digital print.
c) Melamine Formaldehyde Resin.
A basic substance in Laminate and WFF floors is the thermosetting melamine formaldehyde resin that is used as a binder. Melamine resin or melamine formaldehyde resin (generally shortened to melamine) is a hard, thermosetting plastic material made from melamine and formaldehyde by polymerization. Such resin, hereafter referred to as melamine, comprises three basic stages. The stages, A-stage, B-stage, C-stage are described in Principles of Polymerization, George Odian, 3rd edition, which is hereby incorporated by reference, including particularly pages 122 to 123. The first uncured A-stage is obtained when melamine, formaldehyde and water is boiled to a liquid substance with a dry content of about 50%. The second semi-cured B-stage is obtained when the liquid resin is used to impregnate, for example, an overlay paper that after the application of the liquid resin is dried with heat. The molecules have started to cross link but the resin is still possible to cure in a final stage if the drying of the resin is made during a rather short time, for example, one minute and with a heat of about 90-120° C.
The B-stage may also be obtained by spraying the liquid resin over hot air such that the drops are dried and a dry semi-cured melamine formaldehyde powder is obtained that comprises small round spherical particles with a diameter of about 30-100 microns (0.03-0.10 mm).
The final completely cured C-stage is obtained when, for example, the melamine impregnated paper or the WFF powder is heated to about 160° C. under pressure during 10-20 seconds. The dry melamine formaldehyde resin becomes softer, melts and cures to a fixed form when the temperature increases during the pressing. The curing is dependent on temperature and heating time. Curing may be obtained at lower temperatures and longer time or at higher temperature during shorter time. Spray dried melamine powder may also be cured under high temperature.
d) Wood Floors.
Wood floors are produced in many different ways. Traditional solid wood floors have developed into engineered floors with wood layers applied on a core made of wood lamellas, HDF or plywood. The majority of such floors are delivered as pre-finished floors with a wood surface that is coated with several transparent layers in the factory. The coating may be made with UV cured polyurethane, oil or wax. Recently wood floorings have also been produced with a digitally printed pattern that improves the design of the wood grain structure in wood species that do not have a sufficient surface quality.
e) Ceramic Tiles
Ceramic tiles are one of the major materials used for flooring and wall coverings. The raw materials used to form tiles consist of clay minerals, feldspar and chemical additives required for the shaping process. One common method to produce ceramic tiles uses the following production steps. The raw materials are milled into powder and mixed. Sometimes, water is then added and the ingredients are wet milled. The water is removed using filter pressing followed by spray drying into powder form. The resulting powder is then dry pressed under a very high pressure (about 400 bars) to a tile body with a thickness of 6-8 mm. The tile body is further dried to remove remaining moisture and to stabilize the tile body to a solid homogenous material. Recently dry pressing of large and thin panels have been introduced. Dry granular material is pressed with very high pressure up to 400 bars and panels with a size of 1*2 m and more and with thicknesses down to a few mm may be produced in a cost efficient way. Such panels may be used for wall panels and worktops. The production time has been reduced from several days to less than an hour. Such panels may be cut and shaped with production tolerances that are superior to the traditional methods and the may even be installed in a floating manner with mechanical locking systems. One or several layers of glaze, which is a glass like substance, are applied on the tile body by dry or wet methods. The thickness of the glazing is about 0.2-0.5 mm. There may be two glazes on the tile, first a non-transparent glaze on the tile body, then a transparent glaze on the surface. The purpose of tile glazing is to protect the tile. The glaze is available in many different colours and designs. Some glazes can create different textures. The tile is after glazing fired in a furnace or kiln at very high temperatures (1,300° C.). During firing, the glaze particles cure and melt into each other and form a wear resistant layer. Roller screens are often used to create a decorative pattern. The contact nature of the rotary screen-printing has many disadvantages such as breakages and long set-up times. Several tile producers have therefore recently replaced this conventional printing technology with digital ink jet printing technology that offers several advantages. Generally oil based inks are used and the print is applied on the pressed tile body or on a base glazing that is applied in wet form and dried prior to printing. A transparent glaze layer may be applied on the digital print in order to improve the wear resistance. Digital non-contact printing means no breakages and possibility to use thinner tile bodies. Short set-up times, randomized printing with no repetition effects and ability to print on surfaces of variable structures and on tiles with beveled edges are other major advantages. Additional circumstances that have contributed to the introduction of the digital printing technology in the tile industry is the fact that ceramic tiles are rather small compared to, for example, laminate and powder based floors that are produced as large pressed boards of about 2.1*2.7 m. Rather small printers with limited number of print heads may be used in the tile industry and the initial investment is rather limited. Oil based inks have a very long drying time and clogging of nozzles may be avoided. Other advantages are related to the glazing that provides a base colour. Generally smaller amounts of pigments are required to form a tile pattern on a base colour than to provide an advanced wood grain design on a HDF or paper material used in laminate floorings where impregnation and lamination creates additional problems.
f) LVT Floorings.
Luxury Vinyl Tiles, generally referred to as LVT floorings, are constructed as a layered product. The name is somewhat misleading since a major part of LVT floors have a plank size with a wood pattern. The base layer is made primarily of several individual base layers comprising different mixtures of PVC powder and chalk filler in order to reduce material costs. The individual base layers are generally about 1 mm thick. The base layer has a thin high quality printed decorative PVC foil on the upper side. A transparent wear layer of vinyl with a thickness of 0.1-0.6 mm is generally applied on the decorative foil. Glass fibres are often used to improve thermal stability. The individual base layers, glass fibres, the decorative foil and the transparent layer are fused together with heat and pressure in continuous or discontinuous presses. The transparent layer may include a coating of polyurethane, which provides additional wear and stain resistance. Some producers have replaced the transparent vinyl layer with a polyurethane layer that is applied directly on the decorative foil. Recently new types of LVT floors have been developed with a base layer thickness of 3-6 mm and with edges comprising mechanical locking systems that allow floating installations. LVT floors offer several advantages over, for example, laminate floors such as deep embossing, flexibility, dimensional stability, moisture resistance and lower sound. Digital printing of LVT floors is only on an experimental stage but would, if introduced, provide major advantages over conventional printing technology.
As a summary it may be mentioned that digital printing is used in several floor types to create a décor. However the volumes are still very small, especially in wood and laminate flooring applications, mainly due to high cost of the ink and high investment cost for the industrial printers. The flexibility that the digital printing technology provides is limited by the embossing that is fixed and not possible to adapt to the variations of the digitally printed décor. It would be a major advantage if the ink cost could be reduced, if more cost efficient printing equipment could be used in an industrial scale, if a higher wear resistance could be reached without separate protective layers and if variations in the embossed structures may be formed that correspond to variations in the digitally printed pattern.